class instruments():
    def make_sound(self):
        pass
class Erhu(instruments):
    def make_sound(self):
        print('二胡在演奏')
class Piano(instruments):
    def make_sound(self):
        print('钢琴在演奏')
class Violin(instruments):
    def make_sound(self):
        print('小提琴在演奏')
def play(obj):
    obj.make_sound()
er=Erhu()
piano=Piano()
violin=Violin()
play(er)